Seawater cell with increased efficiency

ABSTRACT

This invention relates to a sea water cell which uses wave action to increase the flow of the water through the cathode. The cell has an anode and a cathode arranged in an open cell structure to allow the electrolyte, i.e. the sea water, to flow through the structure. The cell structure is provided with water flow deflector means causing water flow through the cell to be deflected from the vertical direction, when the cell is suspended from or attached to a buoyancy device. The water flow deflector is preferably arranged in the upper end of the cell.

TECHNICAL FIELD

The present invention relates to sea water cells or batteries which useoxygen dissolved in sea water as oxidants, as for instance cellsdescribed in international patent application Nos. PCT/N089/00040 andPCT/N090/00045.

BACKGROUND ART

The sea water cell described consists of an anode made from anelectronegative alloy e.g. based on magnesium, zinc, aluminum orlithium, and the cathode is a more or less inert current conductor.Common materials in the cathodes are materials which are resistant tosea water, such as copper, stainless steel, titanium or carbon. Thecathode may also be coated with a catalyst which catalyzes the reductionof oxygen. Sea water contains little oxygen, about 10 g/m³. As a resultof this the oxygen reducing sea water cells must have a very openstructure in order to allow sufficient flow of fresh sea water throughthe cathode. Batteries consist usually of cells which are connected inparallel because the cells have the seawater as a common electrolyte.The common electrolyte would give short circuit currents via the seawater in series connected batteries. A DC/DC converter converts the lowvoltage (1 to 2 V) of the sea water cell to a more useful value, as e.g.28 V.

In a battery having magnesium anodes the following reaction will takeplace:

At the anode: 2 Mg=2 Mg²⁺ +4 e⁻

The electrons liberated at the negative electrode are consumed at thepositive electrode (the cathode):

    O.sub.2 +2 H.sub.2 O+4e.sup.- =4 OH.sup.-

The concentration of oxygen in sea water is low, so that the transportof oxygen to the surface of the cathode will be the reaction steplimiting the performance of the battery. Further one has to ensure thatthe surface of the electrode does not become so alkaline that it leadsto deposition of calcium carbonate from the sea water, as this may forma layer on the cathode. Such a layer will, if it is formed, lead to apermanent reduction of the performance of the battery. For this reasonthe alkalization must be limited. This is obtained by limiting thecurrent so that it does not exceed a certain percentage of the limitingcurrent of the cathode. The limiting current is the current density atwhich the concentration of oxygen at the surface of the electrode iszero, in other words where the current density is so high that anyoxygen molecule which is transported to the electrode surface bydiffusion or convection, is reduced by formation of hydroxyl ions. Thecathode is therefore given such a structure that a highest possiblelimiting current is obtained. A different parameter which is often usedin literature is the so called mass transfer coefficient, k_(m), whichin this case is the limiting current density divided by the oxygenconcentration.

The limiting current density increases with increased water velocity,oxygen concentration and temperature and decreases with increased sizeof the cathode element. It has been found to be advantageous to make thecathode from expanded metal, net or metal wool because this can limitthe size of the cathode in the flow direction of the sea water. Thecathode can also be designed so that there is little resistance againstflow-through, so that the sea water within the cathode is renewedcontinuously. These problems and solutions are described in int. pat.appl. PCT/N090/00045.

DESCRIPTION OF INVENTION

The object of the present invention is to increase the efficiency of seawater cells of the above mentioned type. The main features of theinvention are defined in the following patent claims. The inventionmakes use of the vertical movement of the sea water cell to increase thewater flow through the cathode and thereby the maximum current deliveredby the cell.

BRIEF DESCRIPTION OF DRAWINGS

Above mentioned and other features and objects of the present inventionwill clearly appear from the following detailed description ofembodiments of the invention taken in conjunction with the drawings,where

FIG. 1 shows a sketch of a sea water cell,

FIG. 2 shows the principle of the invention,

FIG. 3 shows a light buoy

FIG. 4 shows the embodiment of a cell which is suspended in accordancewith the invention, and

FIG. 5 shows an anode structure.

BEST MODE FOR CARRYING OUT THE INVENTION

FIG. 1 shows a typical sea water cell which is placed in the sea water(the electrolyte). The cell 3 is open in top and bottom and it isprovided with an anode 1 and a cathode 2. At least the cathode 2 has anopen structure so that water can flow through its walls. The flowthrough of the cathode is obtained by the horizontal component of thewater flow. If the cell is moved in the vertical direction as the arrow4 shows, this movement does not lead to formation of pressure gradientsacross the cathode and leads therefore also very little to increasedthrough flow the cathode walls.

In FIG. 2 is shown what happens when the seawater cell is closed in oneend with a lid 5. The lid may be tight, or it may be partly open, and itcan also be provided with valves (not shown) which possibly could becontrolled by the water flow. An upwards movement of the cell will leadto an underpressure within the cell, and water will be sucked in throughthe walls of the cathode from the outside. Correspondingly a movement ofthe cell downwards will lead to an overpressure on the inside of thecell so that water is pressed out through the cathode walls. These waterflow directions are indicated with the arrow 6.

It should be obvious that whether the constriction to flow is located inthe top, the middle or the bottom of the cell, vertical cell movementwill result in an increased transport of seawater through the cathode.But as the reaction product from the anode and biofouling organism asbarnacles and mussels sink in seawater, the preferred location toprevent clogging is in the top of the cell.

It is only the velocity of the seawater, not the direction, thatdetermines the transport of oxygen to the cathode surface as long as theconcentration of oxygen in the bulk of the seawater is unchanged.Therefore, one-way valves (not shown) in the constriction guaranteesfresh seawater, but may do so at the expense of the amount of oxygenmade available to the cathode surface.

The energy for moving the cell up and down in the water can be takenfrom the sea waves by giving the combination of buoyancy device and seawater cell such a shape that the waves 7 (FIG. 3) will move the buoy andthe cell up and down in the sea. FIG. 3 shows employment of theinvention in a light buoy with a cylindrical buoyancy device 8 which inits lower end has about the same diameter as the cathode 2 of the seawater cell. The cell is provided with a flow deflector 5 in its topfacing the buoyancy device 8 to completely or partly deflect theseawater. A light source 10 is mounted on a stand 9 on the buoyancydevice. The converter and the secondary battery (not shown) for poweringthe light source during periods of low sea currents and low wave height,are also mounted in the light buoy.

For certain applications, like in fjords with frequently occurring freshwater layers in the surface, one does not wish to attach the cell or thebattery to the buoy itself, but place it at a greater depth. This isillustrated in FIG. 4. A connection 12 between a buoy 13 and a cell 11should be non-elastic, e.g. consisting of an aramide rope, wire or chainwhile a connection 14 to an anchor 15 should be elastic or flexible,e.g. by using a chain which is substantially longer than the distancebetween the sea water battery and the bottom. As indicated in FIG. 4 itis preferred to give the closure or deflector in the top of the cell 11a rounded form so as not to unnecessarily reduce the vertical movementof the cell.

In the above described examples it has been assumed that the cells havea long life. Here the anode 1 either has the shape of a rod or a pipe inorder to contain sufficient magnesium. For cells where the life time isshort, and where the required amount of magnesium is low, the flowthrough the cell can be further increased by e.g. giving the anode theform of a collection of rods 16 as shown in FIG. 5. The figureillustrates a sea water cell seen from below. Anode rods 16 areinterconnected in the top (not shown) and the connection is preferablymolded into a polymer (not shown). The cathode 17 can as previouslymentioned be a flow-through electrode, e.g. in the form of a helix of anexpanded copper sheet catalyzed with silver. The deflector or closure ofthe cell in the top may be either as shown in FIG. 3 with a buoyancydevice or as shown in FIG. 4 with a rounded top. By making the anodes asmentioned, the open area of the bottom of the cell is increased. At thesame time the flow through the anode is increased. Both these remediesadd to increased flow through the cathode. Anodes for short life timebatteries, i.e. batteries or cells requiring little magnesium, can beformed may also be formed as a perforated tube, or as a cylinder ofexpanded metal.

The above detailed description of embodiments of this invention must betaken as examples only and should not be considered as limitations onthe scope of protection.

I claim:
 1. Sea water cell using oxygen dissolved in sea water as anoxidant, said cell comprising:an anode, a cathode having an openstructure surrounding an open interior to allow sea water to flowhorizontally through the cathode between said open interior and avertical exterior surface of said cathode, a horizontal opening betweensaid open interior and a horizontal exterior surface of said cell,suspension means for inducing vertical movements to the cell in responseto wave action of the sea water on a buoyancy device, to thereby inducea vertical flow of sea water through the horizontal opening, and waterflow deflector means associated with the open interior for causingsubstantially all of the vertical flow of sea water through thehorizontal opening to be deflected to a substantially horizontal flow ofsea water through the cathode, wherein substantially all the sea waterflow through the cathode is induced by vertical movements of the celland flows horizontally through the cathode.
 2. Cell according to claim1, wherein the suspension means comprises a mechanical connectionbetween the cell and the buoyancy device.
 3. Cell according to claim 1,wherein the water flow deflector means is disposed at an upper end ofthe cell.
 4. Cell according to claim 3, wherein the deflector meanscomprises a lid covering at least part of a second horizontal openinginto the cell.
 5. Cell according to claim 1, wherein the deflector meansis at least partly structurally integrated with the anode and cathode.6. Cell according to claim 5, wherein the anode has an open structurethrough which the sea water may flow as it is deflected by the deflectormeans.
 7. Cell according to claim 6, wherein the anode consists of anumber of vertical rods arranged in parallel, thereby increasing thehorizontal water flow through the cathode when the cell is movedvertically.
 8. Cell according to claim 1, wherein the cell is suspendedabove a sea bed and the suspension means comprises a flexible connectionbetween the cell and the sea bed.
 9. Cell according to claim 2, whereinthe mechanical connection is a non-elastic connection.
 10. Cellaccording to claim 9, wherein the deflector means is disposed at anupper end of the cell.
 11. Cell according to claim 10, wherein thedeflector means comprises a lid covering at least part of a secondhorizontal opening into the cell.